I. Field of the Invention
This invention relates to a heated pressure fusing apparatus as is commonly used in xerographic copying machines, and more particularly to a heated fusing apparatus for fusing toner images on a support surface such as a sheet of paper which apparatus is capable of operating with thermal efficiency.
II. Description of the Prior Art
In a typical xerographic process a photoconductor comprising a photoconductive composition coated on a rigid or flexible substrate is uniformly electrostatically charged in the dark and then exposed by being illuminated in an image pattern in accordance with graphic material on an original document. The photoconductor becomes discharged in the areas exposed to the illumination, but retains its electrostatic charge in the dark areas, which areas correspond to the graphic material on the original document. The resulting electrostatic latent image is developed by depositing on the photoconductor a finely divided electrostatically attractable developing material (toner). The toner will normally be attracted to those areas on the photoconductor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This visible image of developing material is then transferred to a support surface, such as plain paper or any other suitable substrate, to become the ultimate copy. Any residual developing material remaining on the photoconductor is removed and the photoconductor is reused as described above for subsequent copies. The toner image that was transferred to the plain paper is then fixed thereto. Since the developing material is heat fusible, application of sufficient heat to the paper causes the developing material to melt and be fused into the paper so as to be permanently affixed thereto.
One very basic approach to fusing in a xerographic copying machine is the use of the so-called heated pressure fusing apparatus. Typically, in this apparatus, the paper with the toner image thereon is passed between a pair of opposed and cooperating rollers, at least one of which is heated. Generally, the heated roll is formed of a hollow cylinder having a radiant heater, such as an infrared lamp or a halogen lamp, centrally located within the cylinder to heat the roll, in series with a bimetal thermostat. A typical example of this type of heated fuser roll is illustrated in U.S. Pat. No. 3,637,976. During operation of the fusing apparatus, the paper to which the toner images are electrostatically adhered is passed through the nip formed between the rolls with the toner image contacting the fuser roll to effect heating of the toner image within the nip. Fusing is enhanced by the second roll or pressure roll as it is commonly called as the result of a biasing force which forces the rolls into engagement. The thermostat intermittently interrups the current flow as the roll temperature reaches a predetermined value. The roll then cools to some lower temperature whereupon the thermostat restores the current, and the roll heats up again.
There are, however, many problems that occur with the type of internally heated pressure fusing apparatus as described above. For example, in many of the known hot-roll fusers it is extremely difficult to maintain a constant temperature at the nip of the rollers where the actual fusing of the toner occurs, and where temperature control is critical. Temperature control is difficult because (1) it is difficult to sense the temperature in this region: (2) thermal lag, i.e., the responsiveness of roll temperature under varying demands of thermal output; and (3) there are both different machine modes, i.e., standby, off, continuous operation, and different size papers to contend with. In addition, internally heated fuser rolls generally require the use of very high heating temperatures for the internal heating elements to enable the outer surface roll temperature in the nip of the rollers to be high enough to melt the toner. The use of these high temperatures can result in deterioration of the fuser roll. Furthermore, and in general, the thermal efficiency of known heated-pressure fusing devices is fairly low.
Examples of other miscellaneous type heated rolls exhibiting many of the problems as outlined above are illustrated in U.S. Pat. Nos. 3,471,683, 3,720,808 and 4,100,397.